1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device, and more specifically, to a method of manufacturing a semiconductor device in which adjacent field-effect transistors are isolated by a trench and to a semiconductor device manufactured by this method.
2. Description of the Background Art
In recent years, demand for semiconductor devices is rapidly growing. Accordingly, development in technology for improving the reliability of a semiconductor device is being pursued. Among the semiconductor devices, for those having a plurality of field-effect transistors, a technique is known for forming a trench for isolating adjacent field-effect transistors.
FIGS. 11 to 17 are cross sectional views illustrating a conventional method of manufacturing a semiconductor device. FIG. 18 is a plan view showing a conventional semiconductor device. FIG. 19 is a cross sectional view taken along the line XIXxe2x80x94XIX in FIG. 18. As shown in FIG. 11, a silicon oxide film 2 is formed on a main surface 1f of a silicon substrate 1. A silicon nitride film 3 is formed on silicon oxide film 2. A resist pattern 4 is formed on silicon nitride film 3. Silicon nitride film 3 is etched using resist pattern 4 as a mask to form an opening 3h. Opening 3h is formed with a side surface 3s of silicon nitride film 3. Moreover, silicon oxide film 2 is also removed.
As shown in FIG. 12, silicon substrate 1 is etched using resist pattern 4 and silicon nitride film 3 as a mask. Thus, a trench 1h having a sidewall 1a is formed.
Then, as shown in FIG. 13, sidewall 1a of trench 1h is oxidized with an oxidizing atmosphere of a high temperature to form a silicon oxide film 1b. At this time, a surface of silicon nitride film 3 is not oxidized.
As shown in FIG. 14, a silicon oxide film 5 is formed so as to fill trench 1h and to cover silicon nitride film 3.
As shown in FIG. 15, a portion of silicon oxide film 5 is removed using CMP (Chemical Mechanical Polishing) method. Consequently, an upper surface of silicon nitride film 3 is exposed.
Then, as shown in FIG. 16, silicon nitride film 3 is removed with hot phosphoric acid. As a result, an upper surface of silicon oxide film 2 is exposed.
As shown in FIG. 17, silicon oxide film 2 is removed using a hydrofluoric acid solution. At this time, portions of silicon oxide film 5 filling trench 1h in regions close to silicon oxide film 2 are largely etched from the lateral direction. Consequently, lower portions 5b having a surface height that is relatively lower than other portions are formed.
As shown in FIGS. 18 and 19, a gate oxide film 11 is formed on silicon substrate 1. A conductive layer is formed on gate oxide film 11, and is etched to form a gate electrode 12. Impurities are implanted into silicon substrate 1 using gate electrode 12 as a mask to form a source region 21s and a drain region 21d. In this manner, field-effect transistors 100a and 100b are completed. Adjacent field-effect transistors 100a and 100b are isolated from one another by silicon oxide film 5 in trench 1h. 
A problem that arises in the conventional method of manufacturing a semiconductor device will be described below.
According to the conventional manufacturing method, lower portion 5b is formed in silicon oxide film 5. Since the surface level of lower portion 5b and other portions differs so that a thickness of gate oxide film 11 in the vicinity of lower portion 5b and a thickness of gate electrode 12 formed thereon become non-uniform, as shown in FIG. 18. As a result, such problem as threshold voltage fluctuation occurs, which leads to degradation in reliability of the semiconductor device.
The present invention was made to solve the above-described problem, and its object is to provide a highly reliable semiconductor device and a method of manufacturing such a device.
A method of manufacturing a semiconductor device according to the present invention includes a step of forming an underlying layer and a first layer having an opening on a main surface of a semiconductor substrate. The opening is formed by the underlying layer and a side surface of the first layer. The method of manufacturing the semiconductor device further includes a step of forming a groove having a sidewall that is continuously formed with a side surface of a first layer and the underlying layer by etching the semiconductor substrate using the first layer as a mask, a step of altering quality of the side surface of the first layer to form an altered layer in contact with the underlying layer, a step of forming a second layer for filling the groove such that it comes in contact with the altered layer, a step of removing the first layer, with the altered layer in contact with the second layer remaining, and a step of removing the underlying layer with the second layer filling the groove after removal of the first layer.
According to the method of manufacturing a semiconductor device having such a step, the second layer is protected by the underlying layer when the underlying layer is removed so that the second layer is not removed by a large extent. Thus, a portion that is lower than a main surface of the semiconductor substrate is not formed on an upper surface of the second layer. As a result, even when a field-effect transistor is formed on the upper surface of the second layer and on the main surface of the semiconductor substrate, characteristics of the field-effect transistor would not be degraded and reliability of the semiconductor device can be improved.
Moreover, the step of forming an altered layer preferably includes altering quality of a side surface and an upper surface of the first layer to form an altered layer.
In addition, the step of removing the first layer preferably includes removing the first layer by etching such that an etch rate for the first layer is higher than an etch rate for the altered layer and the second layer. In this case, the etch rate for the first layer is relatively high while the etch rate for the altered layer and the second layer is relatively low so that etching of the altered layer and the second layer can be prevented.
Further, the first layer preferably includes a silicon nitride film, while the underlying layer, the altered layer, and the second layer preferably include silicon oxide films.
In addition, the semiconductor substrate is preferably a silicon substrate, and the method of manufacturing a semiconductor device further includes a step of forming an underlying layer made of a silicon oxide film on a main surface of the semiconductor substrate prior to forming the first layer. The step of forming the first layer includes forming the first layer on the underlying layer. In this case, a silicon oxide film exists between a silicon substrate and a silicon nitride film so that characteristics of the semiconductor device would not be degraded.
Moreover, the step of forming the altered layer preferably includes oxidizing a side surface of the first layer.
Further, the step of forming the altered layer preferably includes oxidizing a side surface of the first layer and a sidewall of a groove. In this case, the sidewall of the groove can be oxidized to eliminate a defect on the sidewall of the groove while the altered layer is formed at the same time so that the altered layer can be formed without an additional step.
More preferably, the step of oxidizing the side surface of the first layer includes oxidizing the side surface of the first layer using a dinitrogen oxide gas. In this case, oxidation of the side surface of the first layer formed by a silicon nitride film can be ensured using a dinitrogen oxide gas.
Furthermore, the step of oxidizing the side surface of the first layer preferably includes oxidizing the side surface of the first layer using a mixed gas of oxygen gas and nitrogen oxide gas. In this case, oxidation of the side surface of the first layer formed by a silicon nitride film can be ensured using the mixed gas.
In addition, the step of oxidizing the side surface of the first layer preferably includes oxidizing the side surface of the first layer using a mixed gas of oxygen gas and hydrogen gas. In this case, oxidation of the side surface of the first layer formed by a silicon nitride film can be ensured using the mixed gas.
Moreover, the step of forming the second layer includes forming the second layer that fills a groove and covers the first layer. The method of manufacturing the semiconductor device further includes a step of removing the second layer covering the first layer prior to removal of the first layer.
The semiconductor device according to the present invention is manufactured using any one of the above-described methods. In such semiconductor device, a portion in which an upper surface of the second layer is lower than a main surface of the semiconductor substrate is not formed so that high reliability of the semiconductor device can be achieved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.